An AC generator is well known which has field or exciting winding means supplied with a rectified exciting current from a single phase or multiple phase magnet field type AC generator. The excited AC generator will be referred to as main generator and the magnet field type AC generator will be referred to as exciter, hereinafter. Such main generator has usually employed a voltage regulator comprising thyristor means serving to partially short the AC output or exciting power from the exciter under control responsive to the signal detected from the main generator. In the prior arts, such detection of the output of the main generator has been effected by a voltage divider including linear resistance means connected to the output of the main generator. When the signal reaches a predetermined level, thyristor means is adapted to be turned on for shorting the exciting current from being supplied to exciting winding means. However, because of such voltage divider detecting a signal of the substantially same waveform as that of the output voltage of the main generator, when it is applied with electrical load so that the load current flows from the main generator to the load, the voltage regulator has never been capable of properly regulating the output voltage of the main generator. More particularly, where the main generator is under no load, thyristor means is to be turned on in an advanced phase of a cycle at more than predetermined level of the output of the main generator, but where the main generator is under load, thyristor means is required to be turned on in a relatively delayed phase at predetermined level of the output of the main generator. Nevertheless, the prior voltage regulator tends to turn on thyristor means in the advanced phase even where the main generator is under load because the detected signal has a steep rise of the curve due to its waveform being substantially identical to that of the output of the main generator. Thus, the conventional voltage regulator has the disadvantage of the output of the main generator under load decreasing as the load current becomes larger, resulting in improper regulation of the output of the main generator.